In general, a roll rigidity control apparatus for a vehicle, such as an automobile, which apparatus includes roll rigidity changing means, such as an active stabilizer apparatus, at a front wheel position and a rear wheel position, is configured to compute a target-roll control variable of the entire vehicle from lateral acceleration of the vehicle, distribute the target-roll control variable of the entire vehicle to front and rear wheels on the basis of a front-rear distribution ratio of roll rigidity to thereby compute target-roll control variables for the front wheel position and the rear wheel position, and control the roll rigidity changing means at the front wheel position and the rear wheel position on the basis of the target-roll control variables for the front wheel position and the rear wheel position.
Such a roll rigidity control apparatus is already known and disclosed in Japanese Patent Application Laid-Open (kokai) No. 2006-21594 for the application filed by the applicant of the present application. The roll rigidity control apparatus disclosed in the publication estimates for the front and rear wheels the degrees of margin for generation of lateral force, computes a ratio of distribution of roll rigidity to the front and rear wheels such that it reduces the magnitude of a difference in degree of margin for generation of lateral force between the front and rear wheels, and distributes a target-roll control variable of the entire vehicle based on lateral acceleration of the vehicle to the front and rear wheels on the basis of the roll rigidity distribution ratio, to thereby compute target-roll control variables for the front wheel position and the rear wheel position. This previously proposed roll rigidity control apparatus can more properly control the roll rigidities at the front wheel position and the rear wheel position in accordance with the degrees of margin for generation of lateral force of the front and rear wheels, as compared with a conventionally known general roll rigidity control apparatus in which the front-rear distribution ratio of roll rigidity is previously set.
In general, when a vehicle turns, the vehicle revolves about the center of turn, and rotates in a yaw direction about the centroid of the vehicle. Therefore, lateral accelerations at the front wheel position and the rear wheel position differ from that at the centroid. Further, the centroid of the vehicle is typically located forward of the geometric center between the front wheel position and the rear wheel position, and the lateral accelerations at the front wheel position and the rear wheel position differ from each other. The difference between these lateral accelerations changes depending on the state of turn motion of the vehicle, and in particular, changes depending on the state of rotation of the vehicle even when the revolution radius at the time of turning is the same.
Therefore, the above-described conventionally known general roll rigidity control apparatus and the above-described previously proposed roll rigidity control apparatus cannot control the roll rigidities at the front wheel position and the rear wheel position while sufficiently taking into consideration that lateral acceleration differs between the front wheel position and the rear wheel position and that the difference in lateral acceleration changes depending on the state of turn motion of the vehicle. Accordingly, the conventionally known roll rigidity control apparatus and the proposed roll rigidity control apparatus have a problem in that they cannot necessary perform optimal roll rigidity control in accordance with the state of turn motion of the vehicle.
Further, the above-described conventionally known general roll rigidity control apparatus and the above-described previously proposed roll rigidity control apparatus premise that a vehicle includes roll rigidity changing means at each of the front wheel position and the rear wheel position. Therefore, for a vehicle which includes roll rigidity changing means only at the front wheel position or the rear wheel position, studies have not yet been performed on a manner of controlling the roll rigidities at the front wheel position and the rear wheel position in consideration that lateral acceleration differs between the front wheel position and the rear wheel position and that the difference of lateral acceleration changes depending on the state of turn motion of the vehicle, as described above.